This invention relates to a magnetic recording medium for use in recording information at a high density and partcularly to a method of manufacturing the magnetic recording medium. It is to be noted throughout the instant specification that a magnetic disk will be mainly described as an example of the magnetic recording medium hereinunder and will be included in a magnetic disk device, although this invention is not restricted to the magnetic disk or the magnetic disk device.
A magnetic disk of the type described has a great number of tracks on which information is linearly recorded or read out by sliding or flying a magnetic head over the magnetic disk along a circumferential path.
A recent requirement for such a magnetic disk is to record information on the magnetic recording disk at a high recording density. In order to accomplish a high recording density, consideration must be made about a linear density and a track density on the magnetic disk, although description will be mainly directed to the linear density hereinunder.
It is to be noted that the linear density is correlated to a flying height of the magnetic head, namely, a distance between the magnetic head and the magnetic recording medium. More specifically, the flying height should be reduced as such as possible with an increase of the linear density. A recent requirement is that the magnetic head flies over the magnetic disk at a flying height equal to or less than 1500 angstroms during recording or reproducing information on the magnetic disk. In order to render the flying height equal to or less than 1500 angstroms, it is necessary that contact never takes place between the magnetic head and projections remaining on the surface of the magnetic disk even when the magnetic head flies over the magnetic disk at a flying height equal to or less than 1000 angstroms.
Moreover, a further reecent proposal has been offered about recording and reproducing information on a magnetic disk by bringing a magnetic disk into contact or near contact with a magnetic disk, so as to further improve linear density. Such a recording or a reading method will be collectively called contact recording. In this event, the flying height of the magnetic head is less than 1000 angstroms. Specifically, the flying height is perferably equal to or less than 760 angstroms (3 microinches).
In order to carry out the contact recording, the magnetic disk itself should have an excellent mechanical durability and an extremely uniform disk or medium principal surface because the magnetic head is brought into contact with the magnetic disk now and then. In addition, when the flying height of the magnetic head is reduced to 760 angstroms, it often happens that the magnetic head objectionably contacts the magnetic disk due to vibration or mechanical shock imparted from an external device, such as an actuator, while the magnetic head is traveling over the magnetic disk.
In this connection, it is very important that the magnetic disk for such contact recording does not have, on the disk principal surface, any irregular projections greater than the flying height of the magnetic head.
Herein, it is noted that collision between the projections and the magnetic head can be evaluated by a glide test which is carried out by making the magnetic head glide over the medium or disk principal surface with a distance left between the magnetic head and the disk principal surface. It is said that the magnetic disk for contact recording must pass the glide test carried out at a flying height which is less than 1000 angstroms and which may be preferably 760 angstroms.
A conventional magnetic disk is disclosed in Japanese Unexamined Patent Publication No. Syo 52-20804, namely, 20804/1977 and will be referred to as a first conventional magnetic disk. The first conventional magnetic disk comprises a substrate of an aluminum alloy, a nonmagnetic layer of Ni--P on the substrate, a magnetic layer of Co--Ni--P on the nonmagnetic layer, and a protection layer on the magnetic layer. The protection layer is effective to protect the magnetic layer from a head crash, abrasion of the magnetic layer, and the like. In such a first conventional magnetic disk, the protection layer is formed by applying a tetrahydroxy silane solution on the magnetic layer and thereafter heating the solution. The protection layer is 1,000 angstroms thick and may be composed of a polysilicate layer which is active.
Inasmuch the substrate is composed of the aluminum alloy in the first conventional magnetic disk, it is very difficult to completely remove projections from a substrate principal surface even no matter how the substrate is polished. As a result, the projections which have heights between 1,000 and 2,000 angstroms are inevitably left on the substrate. When the nonmagnetic layer, the magnetic layer, and the protection layer are successively formed on such a substrate having the projections, surface projections which exceed 1,000 angstroms inevitably remain on the disk principal surface. Accordingly, when subjected to the glide test which is less than 1000 angstroms and which may be carried out, for example, at the flying height of 760 angstroms, the first conventional magnetic disk can not pass the glide test. This shows that it is impossible to lower the flying height to a height lower than 1000 angstroms on recording or reading information.
It is mentioned here that a distance between the magnetic layer and the magnetic head should be reduced so as to establish a high recording density. In this connection, it is preferable that a thickness of the protection layer is equal to or less than 500 angstroms. In particular, the thickness of the protection layer is preferably equal to or less than 200 angstroms.
As regards the first conventional magnetic disk, the protection layer is as thick as 1,000 angstroms, as mentioned before, Such a thick protection layer makes the distance between the magnetic layer and the magnetic head objectionalby great and makes it difficult to record information at a high recording density on the magnetic disk.
In addition, the polysilicate layer which is used as the protection layer has no property of reducing friction between the magnetic disk and the magnetic head when the magnetic head is brought into contact with the magnetic disk. Therefore, the protection layer of polysilicate is low in abrasion resistance.
Inasmuch as the protection layer is formed by an active polysilicate layer directly attached to the magnetic layer, the active polysilicate layer adversely affects the magnetic layer to deteriorate characteristics of the magnetic layer and to thereby degrade recording and reproduction characteristics. Moreover, the polysilicate layer has a low ability of preventing water from entering the magnetic layer. Therefore, a lot of defects take place on the magnetic disk and reduce characteristics of the magnetic disk due to intrusion of water into the magnetic layer.
Another conventional magnetic disk is disclosed in Japanese Unexamined Patent Publication No. Syo 61-73227, namely, 73227/1986, and will be referred to as a second conventional magnetic disk. More particularly, a substrate of aluminum is prepared on which a nonmagnetic layer of Ni--P and a magnetic layer of Co--Ni--P are successively deposited on the aluminum plate. On the magnetic layer, a protection layer is formed by coating, a tetrahydroxy silane solution including hard minute particles and by heating the tetrahydroxy silane solution. The protection layer formed in the above-mentioned manner is composed of a polysilicate layer and the hard particles dispersed into the polysilicate layer. Finally, a lubricant layer of perfluoro polyether is formed on the protection layer.
With this structure, the abrasion resistance is improved as compared with the first conventional magnetic disk and the lubricant layer is desirably attached to the protection layer because the hard minute particles are dispersed into the polysilicate layer.
However, it is difficult to avoid occurrence of projections which have heights greater than 1000 angstroms because of the use of the substrate of aluminum. In addtion, the second conventional magnetic disk uses a substrate which has an average roughness (Ra) of about 200 angstroms, prescribed by Japanese Industrial Standard, namely JIS. Inasmuch as such an average roughness of about 200 angstroms substantially falls within a range between 1000 and 2000 angstroms when the average roughness is expressed in terms of a maximum height (Rmax) determined by JIS, the second conventional magnetic disk is not suitable for contact recording.
In addition, since the polysilicate layer is used as the protection layer in the second conventional magnetic layer also, the magnetic layer can not be sufficiently protected by the protection layer, as mentioned in conjunction with the first conventional magnetic disk.